Running Rings Around the Galaxy

California Institute of Technology News Release

2007 May 30

HONOLULU, Hawaii--An astronomer at the California Institute of Technology has
discovered three giant stellar streams arcing high over the Milky Way. Remnants
of cannibalized galaxies and star clusters, the streams are between 13,000 and
130,000 light-years distant from Earth and extend over much of the northern sky.
The new results are being presented by Carl Grillmair at this week's meeting of
the American Astronomical Society in Honolulu, Hawaii.

Two of the newly discovered streams are almost certainly the remains of ancient
star clusters. Known to astronomers as globular clusters, these giant stellar
cities contain between tens of thousands and millions of stars. Though only
about 150 globular clusters orbit the Milky Way today, they may once have
numbered in the thousands.

Over billions of years, the relentless gravitational stresses inflicted on them
by our galaxy have slowly torn them apart, leaving behind long, thin streams of
stars. Once crowded so closely together that they could sometimes actually
collide, these stars are now separated by many light-years, trailing one another
at half a million miles an hour through the dark and lonely reaches of the
galactic halo.

Grillmair, an associate research scientist at Caltech's Spitzer Science Center,
found the streams by analyzing data from the Sloan Digital Sky Survey. Conducted
over several years at the Apache Point Observatory in New Mexico, this survey
has yielded measurements for some 70 million stars spread over one quarter of
the whole sky.

By comparing the colors and brightnesses of the stars to those in surviving
globular clusters, Grillmair was able to isolate stars into groups of similar
age and distance. As he examined how these groups are distributed across the
sky, the giant stellar streams quickly became apparent.

The third stream discovered by Grillmair is spread over a much larger region of
the sky, and is most likely the scattered remains of a dwarf galaxy. Such dwarf
galaxies may contain up to 100 million stars, along with sometimes substantial
amounts of mysterious "dark matter." While the Milky Way galaxy currently hosts
a family of 20 or so known dwarf galaxies, scientists who study the growth of
galaxies in the early universe have long been puzzled as to why we don't see
hundreds of them.

The new dwarf galaxy stream is particularly interesting in that Grillmair may
also have found the feebly struggling remains of the original galaxy.

The stripping and cannibalization of dwarf galaxies and star clusters can be
quite episodic, resulting in streams with strong concentrations of stars
separated by often substantial gaps, much like the typical distribution of cars
on a highway. However, one of the stellar concentrations is many times stronger
than the others, and Grillmair believes this may be the remnant of the galaxy
that gave rise to the stream. "We'll need to bring out the big telescopes to
examine these stars in detail and find out whether they're still gravitationally
bound to one another," he says.

The new streams are among the largest features in the sky. The streams extend to
the limits of the Sloan survey area, and Grillmair considers it quite likely
that they wrap completely around the galaxy.

Grillmair is now planning follow-up observations of stars in these streams.
While this discovery strengthens the notion that there may be thousands of such
streams and that the outer reaches of our galaxy look more like a "ball of
yarn," the real payoff may be in using these streams as very sensitive probes of
galactic gravity.

By measuring the velocities of stars in the streams, astronomers can both map
their orbits and determine how matter is distributed in our galaxy. "It's a bit
like looking at aerial TV coverage of kayakers on a river," says Grillmair.
"The location of the river immediately tells you where the bottom of the valley
and the major rock outcroppings are, and the speed of the kayakers gives you
some idea of how high and how steep the surrounding mountains must be."

The new discoveries bring to nine the number of known stellar streams around the
galaxy. These streams are helping astronomers study our galaxy's underlying
structure more accurately than ever before.

Overall, the streams appear to be consistent with a picture in which our galaxy
is dominated by huge amounts of smoothly distributed dark matter. On the other
hand, Grillmair and others have already found intriguing departures that may be
showing up cracks in this model.

"This is a very exciting time for galactic archeology, and finding more of these
ancient streams will really help us to piece together the structure of our
galaxy and how it evolved over time."